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Schneider Sepam Protection Relay – Sepam 20 and Sepam S40 Technical Overview

Schneider Sepam is one of the most established and reliable digital protection relay families offered by Schneider Electric for medium voltage applications. The Sepam range has been widely adopted across industrial plants, utilities, infrastructure projects, and commercial facilities due to its proven performance, modular design, and ease of use.

Among the Sepam series, Sepam 20 (also referred to as Sepam S20) stands out as the ideal solution for standard and common medium voltage protection requirements. It provides a perfect balance between functionality, reliability, and cost-effectiveness for typical distribution network applications.

What is Schneider Sepam Protection Relay?

Schneider Sepam protection relay is a microprocessor-based digital relay designed to protect, control, and monitor electrical equipment in medium voltage systems. It continuously measures current and voltage signals from instrument transformers, analyzes them according to pre-set protection functions, and issues trip commands to circuit breakers when abnormal conditions are detected.

The Sepam family includes several application-specific models. Each model is optimized for a particular type of equipment or protection requirement, making selection straightforward for engineers and panel builders.

Sepam Series 20 (Sepam 20 / Sepam S20) Overview

Sepam 20 is the entry-to-mid level range within the Sepam family. It is specifically designed for standard applications in substations, feeders, transformers, motors, and generators. Despite being positioned as a standard solution, it offers comprehensive protection functions, advanced metering, and reliable communication capabilities.

Key characteristics of Sepam 20 include:

• Dedicated models for different applications (S20 for feeders, T20 for transformers, M20 for motors, etc.)
• High measurement accuracy and fast response time
• Built-in disturbance recording and event logging
• Self-diagnostic functions for improved reliability
• Modular architecture that allows addition of I/O modules and communication interfaces

Main Protection Functions of Sepam 20

Sepam 20 provides a wide range of protection functions commonly required in medium voltage networks:

• Overcurrent Protection (ANSI 50/51)
• Earth Fault Protection (ANSI 50N/51N)
• Thermal Overload Protection (ANSI 49RMS)
• Undervoltage and Overvoltage Protection (ANSI 27/59)
• Negative Sequence / Current Unbalance Protection (ANSI 46)
• Breaker Failure Protection (ANSI 50BF)
• Automatic Reclosing (ANSI 79)
• Cold Load Pickup and Inrush Blocking

These functions make Sepam relay suitable for both basic and moderately complex protection schemes in distribution systems.

Sepam S40 – Advanced Version for Demanding Applications

While Sepam 20 is ideal for standard applications, Sepam S40 offers enhanced capabilities for more demanding protection requirements. It includes additional protection functions, more advanced logic capabilities, and greater flexibility in communication and I/O configuration.

Sepam S40 is often selected when the application requires more sophisticated protection logic, additional metering features, or integration into larger automation systems.

Key Advantages of Schneider Sepam Relays

Schneider Sepam relays offer several important benefits that have contributed to their widespread adoption:

• High Reliability: Proven performance with hundreds of thousands of units installed worldwide
• User-Friendly Interface: Intuitive front panel and easy-to-use configuration software
• Modular Design: Flexible architecture that can be adapted to different project requirements
• Strong Communication Capabilities: Support for Modbus and IEC 61850 protocols for seamless integration into SCADA and DCS systems
• Comprehensive Diagnostics: Advanced self-monitoring and event recording for faster fault analysis and maintenance
• Excellent Selectivity: Precise coordination with other protective devices in the network

Communication and System Integration

Modern Schneider Sepam relays support multiple communication protocols, enabling easy integration into both new and existing automation systems. Optional communication modules allow connection via serial or Ethernet networks. This capability is particularly valuable in industrial plants and utility substations where centralized monitoring and remote control are required.

Typical Applications of Sepam 20 and Sepam S40

Sepam 20 and Sepam S40 are commonly used in:

• Medium voltage distribution substations
• Industrial plant incoming and outgoing feeders
• Transformer protection in distribution networks
• Motor protection in industrial facilities
• Generator protection in small to medium power plants
• Infrastructure projects such as airports, metro systems, and commercial complexes

Their robust design and reliable performance make them suitable for both indoor and outdoor installations when properly housed.

Why Choose Schneider Sepam Protection Relay?

Engineers and project owners choose Schneider Sepam for several reasons:

• Proven track record with decades of field experience
• Good balance between features and cost for standard MV applications
• Easy configuration and commissioning, reducing engineering time
• Strong after-sales support and availability of spare parts
• Compatibility with Schneider’s broader ecosystem of medium voltage equipment

Whether the project requires basic overcurrent and earth fault protection or more advanced functions with communication capabilities, the Sepam range offers suitable solutions.

Schneider Sepam protection relay, particularly the Sepam 20 and Sepam S40 models, remains a trusted and widely used solution for medium voltage protection. With its application-specific models, comprehensive protection functions, modular design, and reliable communication options, Sepam provides an excellent combination of performance, flexibility, and value.

For standard distribution applications, Sepam 20 offers the ideal balance of features and cost. For more demanding requirements, Sepam S40 provides enhanced functionality while maintaining the same user-friendly approach that has made the Sepam family successful worldwide.

Looking for Schneider Protection Relay for your project?

Looking for reliable Schneider Sepam, Sepam 20, Sepam S40, or complete Schneider relay solutions for medium voltage protection? We offer expert technical support and genuine Schneider Electric products tailored to your distribution and industrial needs.

For quotations and requests: info@electricistanbul.com

Whatsapp: +90 501 076 69 91

Schneider Protection Relay – Comprehensive Guide for Medium Voltage Applications

Schneider protection relay solutions are among the most trusted and widely used in medium voltage electrical networks worldwide. Schneider Electric offers a complete and scalable range of digital protection relays designed for reliable protection, control, and monitoring of feeders, transformers, motors, generators, busbars, and capacitors.

In modern power systems, a protection relay acts as the intelligent brain of the electrical network. It continuously monitors current, voltage, frequency, and other parameters, detects faults quickly, and sends trip commands to circuit breakers. Schneider Electric provides robust, flexible, and communication-ready relays that meet the demanding requirements of industrial plants, utilities, and commercial facilities.

Main Schneider Relay Families by Application

Schneider organizes its protection relays according to specific applications, making it easier for engineers and contractors to select the right solution:

• Feeder Protection Relays — For incoming and outgoing medium voltage lines
• Transformer Protection Relays — Differential, thermal, and backup protection
• Motor Protection Relay — Advanced thermal, unbalance, and locked rotor protection
• Generator Protection Relays
• Busbar and Capacitor Bank Protection
• Overcurrent Protection Relay and Voltage Protection Relay solutions

1. Sepam Series

The Sepam range is a proven, modular, and user-friendly Schneider protection relay family. It has been widely used for many years in transformer, motor, feeder, and generator protection applications.

With models such as Sepam 20, Sepam 40, and Sepam 80, this series offers excellent metering accuracy, comprehensive event recording, disturbance recording, and strong communication capabilities. Sepam relays are known for their reliability and ease of use, making them a popular choice for both new installations and retrofit projects.

2. Easergy Series – P1, P3, P5 (PowerLogic)

The Easergy series represents Schneider’s modern generation of compact and powerful medium voltage protection relays. This family (previously known as Easergy P3 in some markets) is designed for today’s smart grid and industrial requirements.

• Easergy P1: Entry-level protection relay offering simplicity, reliability, and cost-effectiveness. Ideal for basic feeder and motor protection where straightforward functionality is sufficient.
• Easergy P3 (PowerLogic P3): The most popular model in the series. It is a compact yet powerful relay offering more than 40 protection functions, including advanced overcurrent, earth fault, and arc flash protection relaycapabilities. It supports LPCT/LPVT sensors, Ethernet communication, and full IEC 61850 protocol. Easergy P3 provides an excellent balance between features, size, and performance for standard MV applications.
• Easergy P5 (PowerLogic P5): The advanced model designed for more complex and demanding applications. It offers enhanced functionality, multiple Ethernet protocols, embedded web server configuration, and superior performance in large industrial plants and utility substations.

3. MiCOM Series (Easergy MiCOM Px40)

MiCOM (now part of the Easergy MiCOM Px40 series) is Schneider’s high-end, utility-grade protection relay platform. It is designed for transmission, distribution, and critical high-voltage applications.

Models such as MiCOM P14x (feeder), P24x (motor), and other variants provide advanced protection functions including distance protection, differential protection, and interconnection protection. These relays are known for their scalability, high accuracy, and robust performance in harsh environments.

Key Features of Schneider Protection Relays

Schneider Electric relay products stand out with several important advantages:

• Fast and accurate fault detection with high selectivity
• Comprehensive protection functions covering overcurrent, earth fault, differential, voltage, frequency, and arc flash
• Easy configuration and setting using dedicated software tools
• Multiple communication protocols including Modbus, DNP3, and full IEC 61850
• Robust hardware design suitable for harsh industrial and outdoor environments
• Advanced diagnostics, event recording, and disturbance recording
• Support for modern sensor technologies (LPCT/LPVT)

Whether you need a simple overcurrent protection relay, a motor protection relay, or a sophisticated differential protection relay, Schneider offers scalable solutions that can be tailored to your specific application.

How to Choose the Right Schneider Protection Relay

When selecting a Schneider protection relay, consider the following factors:

• Required protection functions (ANSI codes)
• Application type (feeder, transformer, motor, generator, etc.)
• Communication requirements (IEC 61850, Ethernet, etc.)
• Need for arc flash protection
• Sensor type (conventional CT/VT or LPCT/LPVT)
• Panel space and mounting requirements
• Future expansion and integration needs

Schneider’s clear product segmentation (Sepam, Easergy P1/P3/P5, and MiCOM) helps users quickly identify the most suitable relay for their project.

Schneider protection relay solutions provide reliable, flexible, and future-ready protection for medium voltage networks. From the proven Sepam series to the modern and compact Easergy P3 and advanced Easergy P5, Schneider offers a complete portfolio that meets the needs of industrial, commercial, and utility applications.

With strong communication capabilities, advanced protection functions, and excellent build quality, Schneider relays help improve system reliability, reduce downtime, and enhance personnel safety through features such as arc flash protection.

For any medium voltage protection project, choosing the right Schneider relay ensures long-term performance and peace of mind.

Related MV products

• Siemens SIPROTEC 4 Protection Relays – Proven Numerical Protection Technology
• Siemens SIPROTEC 5 Protection Relays – Next-Generation Modular Protection Platform
• Schneider MiCOM Protection Relays – Advanced Solutions for Medium Voltage Networks
• ABB REF615 Protection Relay – Advanced Feeder Protection and Control Solution
• ABB Relion Series Protection Relays (REF601, REX615, REX640)

Looking for Schneider Protection Relay for your project?

Looking for high-performance Schneider protection relay, Schneider relay, Schneider overload relay, or complete medium voltage protection solutions? We provide expert technical support with genuine Schneider Electric products tailored to your requirements.

For quotations and requests: info@electricistanbul.com

Whatsapp: +90 501 076 69 91

A medium voltage (MV) protection relay is the intelligent device that continuously monitors an MV circuit’s current, voltage, frequency and power, detects faults such as short circuits and earth faults, and trips the circuit breaker to isolate the faulty section — protecting transformers, cables, motors and generators and keeping the rest of the network in service.

This pillar guide explains how modern numerical relays work, the ANSI protection functions and relay types, the main relay families we supply, how to select and coordinate a relay, and the governing standards — for MV distribution and industrial projects across the Middle East and Gulf (including Egypt and Saudi Arabia), as well as North Africa, CIS and Sub-Saharan Africa.

What an MV protection relay is (and where it fits)

In power systems, electricity is generated, stepped up for transmission, stepped down at substations and finally distributed to end users. Without protection, a single fault — a short circuit or earth fault — can destroy equipment and cause widespread outages. The protection relay is the “brain” that prevents this: it detects the fault in milliseconds and sends a trip signal to the circuit breaker to clear it. The relay sits inside the medium-voltage switchgear panel, wired to the current and voltage transformers, and it is the component that turns a passive switchboard into a self-protecting system.

How medium voltage protection relays work

Modern MV relays are microprocessor-based (numerical) devices. They receive scaled signals from the current transformers (CTs) and voltage transformers (VTs), sample and analyse them in real time, and compare the measured quantities against pre-set protection settings. When a fault is detected — for example overcurrent, earth fault or undervoltage — the relay decides, based on its programmed logic and time curves, whether to trip the breaker.

Beyond tripping, a modern relay also provides event and disturbance recording, metering, self-supervision and communication (IEC 61850, Modbus, DNP3) for remote monitoring and fault analysis. The ABB REF615, for example, is a dedicated feeder protection relay built on the IEC 61850 standard with transient/intermittent earth-fault protection and a transient disturbance recorder, while the Siemens SIPROTEC 5 platform supports IEC 61850 Edition 2 including 9-2 process-bus applications and integrated arc protection.

Relay technology: from electromechanical to numerical

Understanding the three generations helps when a network has mixed equipment:

• Electromechanical relays — the original technology, using discs and coils. Robust but limited to a single function each, with no communication.
• Static (solid-state) relays — analogue electronics; more functions, still limited.
• Numerical (digital) relays — today’s standard: one microprocessor device carries many protection functions, metering, recording and communication, and is configured entirely in software.

Almost all new MV installations use numerical relays, and the relay families below are all numerical platforms.

Protection functions and ANSI codes

Relay functions are standardised by ANSI/IEEE device numbers (IEEE C37.2) and characterised to IEC 60255. A relay is selected primarily by the protection functions it must provide. The most frequently used MV codes are:

An engineer specifies a relay by listing the ANSI functions the application needs — for example a feeder relay typically needs 50/51 (overcurrent), 50N/51N (earth fault) and often 67 (directional), while a transformer relay adds 87T (differential) and 49 (thermal).

Types of medium voltage protection relays

Relays are grouped by application, each with a typical ANSI set:

• Feeder protection relay — for incoming and outgoing feeders; overcurrent (50/51), earth fault (50N/51N) and directional functions (67/67N). The ABB REF615 is a dedicated feeder relay.
• Transformer protection relay — differential (87T), overcurrent and thermal overload (49).
• Motor protection relay — overload (49), phase unbalance (46), locked-rotor and earth-fault protection.
• Generator protection relay — differential (87G), reverse power, and frequency protection.
• Busbar protection relay — fast differential protection of the busbar zone.
• Arc-flash protection relay — detects an internal arc optically and trips in milliseconds to limit damage and protect personnel.

Protection coordination and relay curves

Overcurrent protection is not just “trip when the current is too high” — it must be graded so that the relay closest to the fault trips first and the upstream relays act only as back-up. This selectivity is achieved with the inverse-time curves defined in IEC 60255: as the fault current rises, the tripping time falls, and the curve shape is chosen from standard inverse (SI), very inverse (VI), extremely inverse (EI) or definite-time characteristics. A downstream feeder relay is set on a faster curve (or lower time multiplier) than the upstream incomer, so a feeder fault is cleared by the feeder relay while the incomer holds back by a coordination margin (typically a few hundred milliseconds).

Directional overcurrent (67) adds another dimension: in a ring or parallel-fed network, the relay must know the direction of the fault current to trip only for faults in its own zone. Differential protection (87) works differently again — it compares the current entering and leaving a protected zone (a transformer, busbar or cable) and trips on any difference, giving fast, selective protection without a time delay. Getting these settings and their coordination right across the whole scheme is the core engineering task, and it depends directly on the network’s fault levels — see short circuit in power systems.

Relay families we supply

We supply and support the main global numerical relay platforms; each has its own in-depth guide:

• ABB — REF615 (dedicated feeder protection, IEC 61850) and the wider Relion family.
• Siemens — SIPROTEC 5 (modular platform) and SIPROTEC 4 (proven numerical technology).
• Schneider Electric — Easergy (P1/P3/P5 series), Sepam (Sepam 20/S40), MiCOM, and the Schneider protection relay overview.

Each family offers different models with varying numbers of protection functions, communication options and I/O configurations; the linked guides give the model-level detail.

Choosing between relay families

With several strong numerical platforms available, the choice between families usually comes down to a few practical factors rather than raw protection capability, since all the leading platforms cover the standard ANSI functions:

• Existing installed base. Matching the relay family already used across a site simplifies spares, settings tools and operator training — a common reason to standardise on one platform.
• Communication and automation. If the substation is built around IEC 61850 with GOOSE interlocking or a process bus, choose a platform with mature IEC 61850 support (for example ABB REF615 or Siemens SIPROTEC 5).
• Application fit. A dedicated feeder relay (REF615) is efficient for a feeder; a modular platform (SIPROTEC 5) suits a substation needing many function combinations from one hardware family; Schneider Easergy, Sepam and MiCOM cover feeder, transformer and motor duties across their ranges.
• Configuration and lifecycle tools. The quality of the setting software, testing support and long-term firmware maintenance is often the deciding factor over a 15–20 year life.

Our role is to match the relay to the protection scheme, the communication architecture and the existing installed base — not simply to the datasheet.

Communication and IEC 61850

Modern MV relays are also communication nodes. IEC 61850 is the dominant substation-automation standard: it standardises how relays exchange data over Ethernet (GOOSE messaging for fast interlocking and protection signalling, MMS for SCADA), and its Edition 2 with 9-2 process bus digitises the CT/VT signals themselves. Relays such as the ABB REF615 (built on IEC 61850) and Siemens SIPROTEC 5 (IEC 61850 Edition 2, process bus) make substation-wide protection, control and monitoring interoperable across vendors. Modbus and DNP3 are also widely supported for legacy SCADA.

How to select and coordinate a relay

Selecting the right relay is only half the job; the settings and coordination are the other half.

1. Required functions. List the ANSI functions the application needs (feeder, transformer, motor…) and choose a relay model that provides them.
2. CT/VT inputs. Confirm the relay’s rated current input (1 A / 5 A), voltage inputs and the number of analogue/digital I/O.
3. Communication. Match the substation’s protocol (IEC 61850, Modbus, DNP3) and redundancy needs.
4. Settings and selectivity. Calculate settings so the relay is selective (coordinated) with upstream and downstream devices — only the faulty section trips, the rest stays in service. This uses time grading, current grading and logic discrimination, with the overcurrent curves defined in IEC 60255 (standard inverse, very inverse, extremely inverse).

Even the best relay will mis-operate if its settings are wrong, so protection coordination is calculated for the whole scheme, not one relay in isolation. For the fault levels that drive these settings, see short circuit in power systems.

Testing, commissioning and the relay lifecycle

A protection relay only protects if it has been correctly set and proven. Before a relay is put into service it is commissioned by secondary injection: a test set injects simulated currents and voltages into the relay’s inputs and verifies that each protection function picks up at the right value and trips in the right time, and that the trip actually operates the circuit breaker. Numerical relays make this far easier than older technology, because the settings, the measured values and the event and disturbance records can all be read out digitally and archived.

Over the relay’s life, the same numerical platform supports self-supervision (the relay continuously checks its own health and raises an alarm on a fault), periodic testing, and firmware updates. When an older electromechanical or static scheme is modernised, a numerical relay retrofit typically replaces a whole panel of single-function relays with one multi-function device, adding IEC 61850 communication, metering and disturbance recording at the same time — a common upgrade path in ageing substations across the region. This lifecycle view matters when specifying: a relay is a 15–20 year asset whose communication and testing capabilities are as important as its raw protection functions.

Standards

MV protection relays are characterised and tested to the IEC 60255 series (measuring relays and protection equipment), their device functions are numbered to IEEE C37.2 (ANSI), and their communication follows IEC 61850 for substation automation. Relays installed in switchgear also operate within the switchgear’s IEC 62271 framework.

Medium voltage (MV) switchgear is the complete assembly of switching, protection, control and measurement equipment used to distribute electrical power at voltage levels above 1 kV and up to about 40.5 kV, built as metal-enclosed or metal-clad panels to international standard IEC 62271-200.

This pillar guide explains the components inside an MV panel, the main types of switchgear — by insulation medium and by construction — how they compare, the classification and ratings you need to specify, the shift toward SF6-free equipment, and how to select the right panel. It is written for MV distribution projects across the Middle East and Gulf (including Egypt and Saudi Arabia), as well as North Africa, CIS and Sub-Saharan Africa.

What MV switchgear is

MV switchgear integrates several essential components into a single, safe, easy-to-operate unit. It provides safe operation, fault isolation, load switching and reliable supply in distribution substations, industrial plants, renewable plants and both step-up and step-down substations. A modern panel is built as a compact, modular metal-enclosed or metal-clad cubicle, and the main components are:

• Circuit breaker — the main switching and fault-interrupting device. Modern MV breakers are almost always vacuum (the arc is interrupted in a vacuum bottle), though SF6 interrupters are also used; the device follows IEC 62271-100.
• Disconnector and earthing switch — provide visible isolation and safe earthing of an isolated circuit; they follow IEC 62271-102. The earthing switch typically has a short-circuit making capacity, and mechanical interlocks make it impossible to earth a live circuit.
• Instrument transformers — current and voltage transformers that scale primary quantities down for metering and protection. See the dedicated guides on the current transformer and voltage transformer.
• Protection relay — the numerical protection relay that measures current and voltage and trips the breaker on a fault, with ANSI/IEC protection functions and IEC 61850 communication.
• Busbars, cable terminations and surge arresters — the busbars distribute current between panels, the cable terminations connect the MV cables (see plug-in cable termination), and surge arresters limit transient overvoltages.
• Low-voltage compartment — houses the control, protection and auxiliary circuits.

Types of MV switchgear — two independent axes

A common source of confusion is treating “GIS”, “AIS”, “metal-clad” and “RMU” as one list of alternatives. In fact they describe two independent things:

• Insulation medium — how the live parts are insulated: Air-Insulated (AIS) or Gas-Insulated (GIS).
• Construction format — how the panel is built and compartmented: metal-clad (compartmented, withdrawable), metal-enclosed cubicle, or ring main unit (RMU) (compact, usually sealed).

So a panel is described by both — for example a metal-clad panel is normally air-insulated, while a ring main unit is normally gas-insulated. The table below summarises the four terms you will meet most often.

Explore each family in depth: gas-insulated switchgear (GIS) ring main units (RMU), metal-clad switchgear and air-insulated switchgear (AIS).

Fixed-mounted vs withdrawable construction

A further construction choice is whether the circuit breaker is fixed-mounted or withdrawable. In a withdrawable (draw-out) design — typical of metal-clad primary switchgear — the breaker sits on a truck that can be racked between the service, test and isolated positions, and fully removed for maintenance or replacement while the busbar and cable compartments stay in their state. This maximises continuity of supply and makes maintenance safe and fast, at the cost of a larger, more expensive panel. In a fixed-mounted design — typical of compact secondary switchgear and gas-insulated RMUs — the breaker is built into the sealed unit; it is simpler and more compact, and because the unit is sealed-for-life it needs little internal maintenance in the first place. The right choice again follows the network level: withdrawable for high-current primary boards where serviceability is paramount, fixed-mounted for compact, sealed secondary units.

Primary vs secondary distribution

Another way to place a panel is by its role in the network:

• Primary distribution switchgear sits close to the source — at the incoming substation feeding several outgoing feeders at high busbar current. It is typically an air-insulated, metal-clad, withdrawable line-up: for example ABB UniGear ZS1 handles up to 24 kV with busbar currents to 4000 A and short-circuit levels to 63 kA (at 12–17.5 kV). Withdrawable breakers allow a faulty unit to be racked out and replaced while keeping the rest of the board in service.
• Secondary distribution switchgear sits deeper in the network, at the many small substations that step MV down to LV. It is typically a compact ring main unit (usually gas-insulated), rated 400–630 A, combining ring switching and transformer protection in one sealed unit — see ring main units.

Choosing the level is usually obvious from the single-line diagram; the more nuanced choices are insulation medium, construction and classification, covered next.

How to choose

The choice between air-insulated and gas-insulated switchgear — and between a metal-clad line-up and a compact RMU — depends on:

1. Space and environment. Space-limited, hot, dusty or humid sites favour compact, sealed GIS/RMU; standard indoor rooms with maintenance access suit AIS.
2. Network level. Large primary substations with high busbar currents use metal-clad, withdrawable AIS; secondary distribution rings use compact RMUs. See ring main units.
3. Maintenance strategy. Sealed-for-life GIS minimises internal maintenance; withdrawable metal-clad maximises serviceability and continuity of supply.
4. Total cost of ownership. GIS has a higher capital cost but lower maintenance and footprint; AIS has a lower initial cost but needs the space and controlled environment.
5. Regulatory constraints. Where SF6-free equipment is required, choose a dry-air or air-insulated family with equivalent ratings (see below).

Classification of MV switchgear

MV switchgear is classified to IEC 62271-200 using three key parameters — the same parameters that appear on every reputable datasheet:

• Internal Arc Classification (IAC): accessibility during an internal-arc fault, given as accessibility type A (authorised personnel) or B (public) and the accessible sides F (front), L (lateral) and R (rear), together with the tested arc current and duration (for example AFLR 20 kA, 1 s).
• Loss of Service Continuity (LSC): how much of the switchgear must be shut down to work on one part — LSC1 (all in one compartment), LSC2A (switching devices accessible with busbars live), LSC2B (both cable and busbar compartments can stay live while working on the device compartment).
• Partition class: PM (metal partitions) or PI (insulating partitions).

Reading a switchgear datasheet

Put together, a datasheet line such as “24 kV, 630 A, 25 kA 1 s, IAC AFLR 25 kA 1 s, LSC2B, PM” tells you: the panel is rated 24 kV and 630 A; it withstands a 25 kA short-circuit for 1 s; in an internal arc it is tested to 25 kA for 1 s with the front, lateral and rear sides protected for authorised personnel; both cable and busbar compartments can stay energised while working on the switching-device compartment (LSC2B); and the partitions between compartments are earthed metal (PM). Learning to read this line is the single most useful skill when comparing offers from different manufacturers.

Typical technical specifications

Typical MV switchgear ratings across the 12–40.5 kV range are:

Air-insulated switchgear needs larger clearances (for 36 kV systems the minimum busbar air clearance is typically 35–36 cm), whereas gas-insulated switchgear achieves the same insulation level in a much smaller volume thanks to the high dielectric strength of the gas.

SF6-free switchgear — the regulatory shift

For decades SF6 was the standard gas for compact MV switchgear, but it is a potent greenhouse gas (GWP roughly 23,000 times CO₂) now restricted by regulations such as the EU F-gas Regulation (EU 2024/573). The market response is SF6-free equipment that keeps the compact, sealed concept using dry / clean air with vacuum interruption — for example ABB SafeRing/SafePlus Air and Schneider SM AirSeT — with a global warming potential of 0 and comparable ratings. When specifying new switchgear, it is worth checking whether the project or utility mandates SF6-free equipment; the gas-insulated switchgear pillar covers the SF6-free options in detail.

Design and safety features

Quality MV switchgear from manufacturers such as ABB, Schneider, Siemens and others integrates: mechanical interlocks to prevent mis-operation; visible isolation and earthing; pressure-relief for internal-arc faults; integrated protection relays with IEC 61850 communication; a modular design for extending the line-up; and conformal coating on electronics for harsh environments. Together these ensure personnel safety, equipment protection and high reliability.

Applications

MV switchgear is used in primary and secondary distribution substations, industrial plants and factories, renewable energy plants (wind and solar), commercial buildings and infrastructure, and mining and oil & gas facilities. The choice between air- and gas-insulated switchgear depends on available space, maintenance strategy, environmental conditions and total cost of ownership.

Standards

Internationally, MV switchgear is designed and tested to the IEC 62271 series. The key parts are:

Individual markets add national specifications (for example TEDAŞ technical specifications in Türkiye), but the IEC 62271 series is the common global baseline. Selecting the right type, the correct classification (LSC and IAC) and a reputable manufacturer ensures long-term reliability, personnel safety and operational efficiency.

Related engineering guides

For related topics, see our guides on what a transformer is, short-circuit and transformer calculation, parallel operation of transformers, hermetically sealed / oil-type transformers, and mobile substations.

Transformer Vector Group and Connection Types (Vector Group of Transformer)

A vector group of transformer, also known as transformer connection group, defines how the primary and secondary windings of a three-phase transformer are connected and the resulting phase shift between them. Understanding the vector group of transformer is essential for correct transformer selection, system design, and especially for parallel operation of transformers.

Three-phase transformers can have their windings connected in different configurations: Star (Y), Delta (D), or Zigzag (Z). These connections determine important electrical characteristics such as neutral availability, harmonic behavior, and phase displacement.

Transformer Connection Types

• Star Connection (Y or y): Windings are connected to a common neutral point. A neutral conductor is available. Denoted with capital “Y” for primary and lowercase “y” for secondary.
• Delta Connection (D or d): Windings are connected in a closed triangle. No neutral point is available. Denoted with “D” or “d”.
• Zigzag Connection (Z or z): Usually used on the secondary side. Each phase winding is split into two halves and connected in a zigzag pattern. This connection provides good performance under unbalanced loads and helps suppress certain harmonics.

The vector group is written with the primary connection first (capital letter), followed by the secondary connection (lowercase letter), and then a number indicating the phase shift.

How Vector Group Numbers Work

The number in the vector group represents the phase angle displacement between primary and secondary voltages in multiples of 30 degrees.

For example:

• Dyn11 → Phase shift = 30° × 11 = 330° (or -30°)
• Ynyn0 → Phase shift = 0°
• Yzn5 → Phase shift = 30° × 5 = 150°

This standardized notation allows engineers to quickly understand the electrical relationship between primary and secondary sides.

Most Common Transformer Vector Groups

• Dyn11 Transformer: The most widely used vector group in distribution networks. Primary: Delta (D) Secondary: Star with neutral (yn) Phase shift: 330° (or -30°)
• Ynyn0: Both primary and secondary are star connected with neutral. Phase shift: 0°.
• Ynd11: Often used in step-up applications. Primary: Star with neutral Secondary: Delta Phase shift: 330°.
• Dy0: Primary delta, secondary star. Phase shift: 0°.
• Yzn5 / Yzn11: Primary star, secondary zigzag. Preferred for low-power transformers (up to 250 kVA) to handle unbalanced loads effectively.

Why is Dyn11 transformer preferred?

• Third harmonic currents circulate within the delta primary and do not appear on the secondary side.
• The star secondary provides a neutral point, which is required for 400V distribution systems in residential and industrial applications.
• Excellent performance under both balanced and slightly unbalanced loads.
• Standard choice for distribution transformers in many countries, including Turkey.

Ynyn0 Transformer Both primary and secondary are star connected with neutral. Phase shift is 0°. This group is often used in transmission systems and when zero phase displacement is required.

Ynd11 Transformer Primary is star with neutral, secondary is delta. Commonly used in step-up applications such as generator transformers in power plants and renewable energy facilities. The neutral on the primary side allows grounding through a neutral resistor for fault current limitation.

Dy0 Transformer Primary delta, secondary star. Phase shift is 0°. Used in some distribution applications where zero phase displacement is needed.

Yzn11 / Yzn5 Transformer Primary star, secondary zigzag. These groups are preferred for low power transformers (typically up to 250 kVA) supplying areas with high single-phase load (such as rural villages or small settlements). The zigzag connection helps balance the load across phases and reduces neutral current issues.

Importance of Vector Group in Parallel Operation of Transformers

One of the most critical requirements for parallel operation of transformer is that all transformers must have the same vector group.

If two transformers with different vector groups (for example, Dyn11 and Yyn0) are connected in parallel, a phase displacement will exist between their secondary voltages. This will cause large circulating currents to flow between the transformers even when there is no external load. These circulating currents lead to overheating, increased losses, and potential damage to the transformers.

Therefore, when planning to connect transformers in parallel, verifying that the vector groups are identical is mandatory.

How to Choose the Right Vector Group

The correct vector group of transformer depends on several factors:

• System requirements for neutral (earthing)
• Need to suppress harmonics (especially 3rd harmonic)
• Whether the transformer will operate in parallel with existing units
• Application type (distribution, step-up, industrial, or rural)
• Local standards and utility specifications

In Turkey and many other countries, Dyn11 is the standard for distribution transformers because it meets both technical and operational needs of modern 400V networks..

The vector group of transformer is much more than just a technical label. It directly affects system compatibility, harmonic performance, neutral availability, and the ability to operate transformers in parallel. Among all groups, the Dyn11 transformer stands out as the most common and practical choice for distribution systems due to its excellent harmonic suppression and neutral provision.

Proper understanding and correct selection of transformer vector groups are essential for safe, efficient, and reliable power system design. Whether for new installations or parallel operation of existing transformers, choosing the right vector group helps prevent operational problems and ensures long-term system stability.

Related MV products

• What is Voltage Transformer?
• Dry Type Transformer: The Complete Technical Guide to Cast Resin Dry Type Transformers
• Transformer Parameters and Equivalent Circuit (Short Circuit Transformer)
• Parallel Connection of Transformers (Parallel Operation of Transformer)

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Parallel Connection of Transformers (Parallel Operation of Transformer)

Parallel connection of transformer, also known as parallel operation of transformer, is a common and effective method used in power distribution systems when additional capacity is required or higher reliability is needed. Instead of replacing an existing transformer with a larger unit, connecting another transformer in parallel often provides a more flexible, economical, and reliable solution.

Transformers can operate individually or in parallel with other units. However, they cannot be connected in parallel under all conditions. Specific technical requirements must be strictly met to ensure safe, efficient, and stable operation. Failure to meet these conditions can result in circulating currents, overheating, reduced efficiency, or even serious equipment damage.

When is Parallel Operation of Transformer Required?

Parallel operation becomes necessary in several situations:

• When the existing transformer capacity is insufficient for increased load demand
• When higher system reliability and redundancy are required (especially in hospitals, data centers, and industrial plants)
• When maintenance flexibility is needed without interrupting power supply
• When load growth is gradual and investing in a single very large transformer is not economical

In such cases, connecting transformers in parallel allows the total load to be shared between multiple units, improving both efficiency and operational security.

Conditions for Parallel Connection of Transformer

For safe and successful parallel operation of transformer, the following technical conditions must be satisfied:

1. Power Ratings The power ratings of the transformers should be equal or within a maximum ratio of 1:3. Large differences in capacity can cause unequal load sharing.
2. Primary and Secondary Voltages The rated primary and secondary voltages of all transformers must be identical.
3. Short-Circuit Voltage (%Uk or %Z) The short-circuit voltage (impedance voltage) values must be the same. Even small differences can lead to unequal current distribution.
4. Operating Frequency All transformers must operate at the same frequency (50 Hz or 60 Hz).
5. Short-Circuit Impedance The difference in short-circuit impedances between parallel transformers should not exceed 10%. Greater differences cause significant circulating currents.
6. Vector Group and Phase Angle The vector groups (such as Dyn11, Yyn0, etc.) and the phase angle displacement between primary and secondary windings must be exactly the same.
7. Polarity The polarities of all transformers must match. Incorrect polarity can cause short circuits.
8. Transformation Ratio The turns ratio (voltage transformation ratio) of all transformers must be identical.

If even one of these conditions is not met, circulating currents will flow between the transformers. These currents do not contribute to the load but cause additional heating and losses, potentially leading to overheating and insulation damage.

Advantages of Parallel Operation of Transformer

Parallel connection of transformer offers several important benefits:

• Higher overall system reliability and redundancy
• Improved energy continuity — if one transformer fails or is taken offline for maintenance, the others continue to supply power
• Better load sharing and higher efficiency at partial loads
• Greater flexibility during maintenance and system expansion
• More economical solution compared to replacing a transformer with a much larger single unit
• Reduced risk of complete power outage in critical facilities

Disadvantages and Risks of Parallel Operation

Despite its advantages, parallel operation also has some drawbacks:

• Possibility of very high short-circuit currents in the system
• Increased complexity in protection coordination and relay settings
• Higher number of auxiliary equipment, cables, and spare parts required
• Greater potential for operational errors during switching and maintenance
• Risk of circulating currents if conditions are not perfectly matched

Therefore, parallel operation requires careful engineering analysis, proper protection design, and expert commissioning.

Technical Considerations for Safe Parallel Operation

When designing a system with parallel operation of transformer, engineers must pay special attention to:

• Accurate calculation of load sharing between transformers
• Proper selection and coordination of protection relays
• Correct sizing of cables and busbars on both primary and secondary sides
• Use of appropriate vector group and impedance matching
• Regular maintenance and periodic testing of all parallel units

Modern protection systems with differential relays and advanced communication can significantly improve the safety and reliability of parallel transformer installations.

Parallel connection of transformer is a widely used and effective solution in power distribution systems when additional capacity or higher reliability is required. However, successful parallel operation of transformer depends on strictly meeting several critical technical conditions, especially regarding voltage, impedance, vector group, and polarity.

When applied correctly with proper engineering and protection, parallel operation provides excellent benefits in terms of reliability, flexibility, and cost-effectiveness. On the other hand, incorrect paralleling can lead to serious operational problems and equipment damage.

For any project involving parallel transformers, detailed technical analysis and compliance with all paralleling conditions are essential for safe and long-term operation.

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Transformer Parameters and Equivalent Circuit (Short Circuit Transformer)

Short circuit voltage is one of the most important parameters of a transformer. It is indicated as %Uk (or %Z) on the transformer’s technical data sheet.

Short circuit voltage (%Uk) is the voltage that must be applied to the primary winding (with the secondary winding short-circuited) to produce the rated nominal current in the primary. This value directly determines the maximum short-circuit current that will flow in case of a fault on the secondary side and is critical for protection device selection.

For distribution transformers:

• Up to 400 kVA → typically %Uk = 4.5%
• Up to 5 MVA → usually between 6% and 8%
• Larger power transformers have higher %Uk values.

Transformer Losses

Transformer efficiency is mainly affected by two types of losses:

• No-load losses (Iron / Core losses): Caused by hysteresis and eddy currents in the magnetic core.
• Load losses (Copper losses): Caused by the resistance of the windings under load.

Reducing these losses is key to achieving high transformer efficiency.

Short Circuit Transformer Current Calculation

The short-circuit current (Ik) of a transformer can be calculated as:

Ik = In / (%Uk / 100)

Where In is the rated nominal current of the transformer.

This calculation must be performed separately for both primary and secondary sides because voltage and current ratings differ.

Transformer Equivalent Circuit

Because the primary and secondary windings are magnetically coupled but electrically isolated, we use an equivalent circuit referred to one side (usually the primary).

The most common model is the T-type equivalent circuit referred to the primary side. It includes:

• r1, x1 → Primary winding resistance and leakage reactance
• r’2, x’2 → Secondary winding resistance and leakage reactance referred to primary
• Rfe → Core loss resistance
• Xm → Magnetizing reactance

These parameters are determined through short circuit test and open circuit (no-load) test.

Transformer Equivalent Circuit

Short Circuit Test (Short Circuit Transformer Test)

In the short circuit test, the secondary winding is short-circuited and a reduced voltage is applied to the primary until rated current flows. This test gives the copper losses and the total impedance of the transformer.

Open Circuit (No-Load) Test

In the open circuit test, the secondary is left open and rated voltage is applied to the primary. This test determines the core losses (iron losses), magnetizing current, and the values of Rfe and Xm.

Apparent, Active, and Reactive Power

• Apparent Power (S) = V × I
• Active Power (P) = S × cosφ
• Reactive Power (Q) = S × sinφ

Understanding transformer parameters and the equivalent circuit is essential for proper transformer selection, protection coordination, and system short-circuit analysis.

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A dry type transformer is a medium voltage power transformer that insulates its windings with solid epoxy cast resin instead of mineral oil, relying on ambient or fan-assisted air for cooling rather than liquid dielectric. Because it carries no flammable insulating fluid, it is the reference technology for indoor substations where fire safety, personnel protection, and low maintenance outweigh the marginal cost premium over oil-filled units. This hub article walks through construction, cooling and insulation classes, sizing logic, applicable IEC and national standards, typical applications, and a side-by-side comparison with oil type transformers, so that procurement and design engineers across the Middle East and Gulf region including Egypt and Saudi Arabia, North Africa, CIS countries, and Sub-Saharan Africa can specify the correct unit with confidence.

What Is a Dry Type Transformer? Definition and Core Principle

In a dry type transformer — also called a cast resin dry type transformer or dry type power transformer — the high voltage windings are cast under vacuum in epoxy resin, forming a solid, void-free insulation block around the conductor. This vacuum-casting process gives the winding excellent dielectric strength, mechanical robustness, and resistance to moisture and chemical attack, without any need for oil-filled tanks, gaskets, or bushings that could leak.

The low voltage winding is produced either with the same cast-resin technique or with prepreg (pre-impregnated) insulation technology, depending on the manufacturer’s design. Unlike oil type units, the windings are not enclosed inside a sealed tank; they remain open to the surrounding atmosphere. This open construction allows straightforward visual inspection of the winding surface during maintenance and also improves natural heat dissipation because the resin block is in direct contact with the cooling air stream.

Dry Type vs Oil Type Transformer: Key Differences

Power transformers are broadly split into two families: oil type transformers and dry type transformers. Oil type units use mineral oil as both the dielectric insulation medium and the cooling fluid. Oil cooling performs very well outdoors and can support compact designs, but the flammable nature of mineral oil introduces an inherent risk of fire and explosion in the event of an internal fault, and any oil leak carries environmental contamination risk.

Dry type transformers eliminate this fluid entirely. Because there is no oil to leak, ignite, or contaminate soil and groundwater, the fire and explosion hazard associated with liquid-filled units is designed out of the equation. The trade-off is manufacturing cost: cast resin dry type transformers are generally more expensive to produce than equivalent oil-filled units, but this premium is routinely justified wherever the installation is indoors and shares space with people — hospitals, schools, commercial towers, and transport infrastructure.

Comparison Table: Dry Type vs Oil Type Transformer

Cooling System: AN and AF Configurations

Dry type transformers rely exclusively on air rather than any liquid medium for cooling, and two configurations are standardized in practice:

• AN (Air Natural): heat is removed purely by natural convection around the resin-cast windings, with no mechanical assistance.
• AF (Air Forced): fans mounted at the bottom of the transformer actively push air through the winding ducts, increasing the thermal transfer rate.

Switching from AN to AF mode is not just a cooling upgrade — it is a genuine capacity boost. When the fans are activated, the same transformer can accept up to 40% additional load compared with its natural-cooling rating. This AN/AF flexibility lets a single physical unit cover two nameplate ratings, which is valuable where a facility’s load profile is expected to grow after commissioning.

Fan activation is not manual — temperature sensors embedded directly in the windings continuously track winding hot-spot temperature and automatically trigger the fans through a relay control system whenever a threshold is reached. This closed-loop thermal management protects the resin insulation from thermal aging while ensuring fans only run when genuinely needed, saving auxiliary power and extending fan service life.

Enclosure and Protection (IP) Ratings

The as-manufactured protection class for a standard cast resin dry type transformer is IP00, meaning the winding assembly itself has no built-in ingress protection and is intended to sit inside a switchgear room or dedicated enclosure. Where the installation is outdoors, or the indoor environment is humid or dusty, the transformer must be fitted inside a protective enclosure rated IP21, IP23, or IP31 to keep out solid objects and dripping/spraying moisture. Selecting the correct enclosure rating is as important as selecting the transformer itself — an IP00 unit placed in an unprotected outdoor yard will fail prematurely regardless of its electrical design margins.

Insulation Classes and IEC 60076-11 Environmental Categories

Cast resin dry type transformers are built with either F class insulation, rated for a maximum operating temperature of 155°C, or the higher-performance H class insulation, rated for 180°C. The choice between F and H class affects both the thermal aging margin of the winding and, in some designs, the achievable overload headroom.

Beyond thermal class, IEC 60076-11 defines three separate classification axes for dry type transformers: environmental class, climatic class, and fire behavior class. The most commonly specified combination in the field is E2–C2–F1.

Environmental Class (E)

Climatic Class (C)

Fire Resistance Class (F)

For projects in humid coastal Gulf sites or high-pollution industrial zones, specifying E2 is typically the safe default, while C1 vs C2 should be matched to the actual minimum ambient/storage temperature the unit will experience on site.

Sizing and Power Ratings

Dry type transformers are manufactured across a wide power range, commonly starting at 250 kVA and extending up to 8 MVA. Within that range, the ratings most frequently selected on real projects cluster around 1600 kVA and 2500 kVA, reflecting typical building and industrial feeder loads.

A practical constraint arises when a dry type transformer is to be installed inside a concrete kiosk substation: ventilation and physical space inside the enclosure typically cap the recommended power at 1250 kVA, in line with TEDAŞ (Turkish state distribution) specifications. This is a useful rule of thumb for any project using compact prefabricated kiosk substations, even outside Turkey, since the underlying constraint — restricted air exchange volume — is a physical one rather than a purely regulatory one.

Selection Criteria Checklist

When specifying a dry type transformer for an indoor or semi-outdoor MV/LV substation, the engineer should confirm the following in order:

1. Power rating — select within the 250 kVA–8 MVA production range, biasing toward proven 1600/2500 kVA platforms where the load profile fits, or de-rating to 1250 kVA maximum for kiosk installations.
2. Cooling mode — decide between AN-only operation or AN/AF dual rating if a 40% future load growth allowance through forced-air cooling is required.
3. Insulation thermal class — F class (155°C) for standard duty, or H class (180°C) where higher thermal margin or compact design is preferred.
4. Environmental/climatic/fire class per IEC 60076-11 — E2-C2-F1 is the common baseline for humid, polluted, or fire-sensitive installations.
5. Enclosure IP rating — IP00 for protected indoor rooms; IP21/IP23/IP31 enclosure required for outdoor or humid conditions.
6. Governing technical specification — confirm compliance with the applicable national distribution utility specification and IEC 60076-11.

Applications

Because dry type transformers eliminate the flammable oil hazard, they are the default choice wherever transformers must sit close to people or sensitive equipment: hospitals, schools, shopping malls, metro stations, data centers, high-rise buildings, airports, and general commercial complexes. Across the Middle East and Gulf region including Egypt and Saudi Arabia, North Africa, CIS countries, and Sub-Saharan Africa, this same logic applies to any indoor substation embedded in an occupied building, transit facility, or dense urban plot where fire code compliance and space constraints make oil-filled units impractical.

Standards and Compliance

Internationally, dry type transformers are designed, tested, and classified according to IEC 60076-11, which governs the environmental (E), climatic (C), and fire behavior (F) classification system described above. In Turkey specifically, any dry type transformer supplied for a project that will be handed over to a distribution company must additionally comply with the TEDAŞ-MLZ/99-031.B technical specification. Buyers outside Turkey should verify the equivalent local distribution-utility specification while treating IEC 60076-11 as the common international baseline. Leading global manufacturers such as ABB and Schneider Electric offer dry type transformer lines with published technical datasheets and catalogue documentation that reference this same IEC framework.

Related guides

See also our guides on medium voltage switchgear, gas-insulated switchgear and ring main units.

An oil immersed transformer is a power transformer in which the windings and core are immersed in insulating mineral oil that simultaneously provides dielectric insulation and heat dissipation for the active parts. This guide explains the two main construction families of oil-type transformers — hermetically sealed units and expansion tank (conservator) units — along with their internal components, cooling methods, protection features, and the criteria buyers should use to select the right unit for medium voltage distribution networks. The content is written for procurement engineers, EPC contractors, and utility buyers sourcing transformers for projects across the Middle East and Gulf region including Egypt and Saudi Arabia, North Africa, CIS countries, and Sub-Saharan Africa, where humid coastal climates, dusty inland environments, and maintenance-limited rural sites all influence the choice of transformer construction.

Why Oil Is Used as the Insulating and Cooling Medium

In an oil immersed transformer, the oil performs two functions at once: it insulates the live conductive parts from each other and from the tank, and it transfers heat generated by copper losses and core losses away from the windings toward the tank walls, where it dissipates into the surrounding air. The oil used is a high-grade mineral insulating oil, specially refined to deliver strong dielectric strength, good thermal stability, and efficient heat transfer. Because the oil is in direct contact with the winding insulation paper, its condition — moisture content, oxidation level, dielectric strength — has a direct impact on the transformer’s insulation life. This is the core reason why the two construction types described below (hermetically sealed and expansion tank) exist: each represents a different strategy for managing the oil’s exposure to the outside atmosphere over the transformer’s service life.

Two Main Types of Oil-Type Transformers

Hermetically Sealed Transformer

A hermetically sealed transformer is filled with transformer oil under vacuum and then completely sealed from the external atmosphere. There is no expansion tank (conservator) open to outside air; instead, the tank is built with flexible corrugated wave walls that expand and contract to absorb the thermal expansion and contraction of the oil as load and ambient temperature change. Because the oil never contacts outside air, humidity, dust, or oxygen, the risks of oil oxidation, moisture absorption, and progressive insulation degradation are significantly reduced. This is currently the most widely used oil-filled transformer design in distribution networks, valued for its lower cost, high reliability, and strong performance in outdoor and harsh operating environments.

Because the system is fully sealed, hermetically sealed transformers do not need an expansion tank, a Buchholz relay, or a silica gel breather — components that conventional open-oil designs require to manage moisture ingress.

Expansion Tank (Conservator) Type Transformer

In an expansion tank type transformer, the main tank is connected to a conservator tank that sits above it and remains open to the atmosphere through a breathing path. As the oil heats up during operation, it expands and flows into the conservator; as it cools, the oil level drops back. Because this is an open system, it requires additional protective components — a Buchholz relay to detect gas accumulation and internal faults, and a silica gel breather to dry the air drawn in and out of the conservator as oil volume changes. Despite these protective devices, moisture and contaminants can gradually enter the oil over the transformer’s life, which can reduce insulation quality and shorten service life compared with a fully sealed design.

Structure and Main Components

A hermetically sealed oil immersed transformer is built from the following core components:

• Copper or aluminum windings
• Grain-oriented silicon steel core
• High-quality mineral transformer oil for insulation and cooling
• Corrugated wave-wall tank (absorbs oil expansion without a conservator)
• Off-load or on-load tap changer
• High voltage and low voltage cable bushings
• Electronic protection relay, typically fitted above 400 kVA
• Pressure relief valve
• Oil temperature indicator with contacts
• Wheels for transportation

Because the hermetic design eliminates the need for an expansion tank, Buchholz relay, and silica gel breather, the overall structure is simpler, which reduces routine maintenance requirements and improves long-term operational reliability.

Cooling Methods

Oil immersed transformers rely on natural or forced circulation of oil and air to remove heat generated by copper and core losses. Hermetically sealed transformers are typically cooled using the ONAN (Oil Natural Air Natural) method, where the corrugated wave walls of the tank increase the effective cooling surface area and allow natural convection of surrounding air to dissipate heat. For higher power ratings or increased load conditions, ONAF (Oil Natural Air Forced) cooling, which adds fans to improve airflow across the tank surface, can be applied to boost heat transfer capacity.

Effective cooling is not a secondary consideration — if internal temperature rises too high, it can damage the paper insulation system and degrade oil quality, directly shortening transformer life.

Cooling Code Reference

Comparison Table: Hermetically Sealed vs Expansion Tank Type

Typical Power Ratings and Applications

Hermetically sealed transformers are commonly manufactured up to 2500 kVA for distribution network applications. They are widely preferred in concrete kiosk substations up to 1600 kVA. These ratings make the design suitable for:

• Public and private distribution substations
• Concrete kiosk-type prefabricated substations
• Industrial plants requiring outdoor, low-maintenance transformers
• Utility networks in humid coastal or dusty inland climates
• Remote or maintenance-difficult project sites where a Buchholz relay and breather servicing routine is impractical

Protection and Monitoring

Above 400 kVA, hermetically sealed transformers are usually equipped with an electronic protection relay. This relay continuously monitors gas accumulation, oil temperature, and internal tank pressure, and issues alarm and trip signals to the medium voltage switchgear to protect the transformer against developing faults before they escalate into failures. This monitoring layer, combined with the pressure relief valve and oil temperature indicator built into the standard component list, gives buyers a layered protection scheme without relying on the mechanical Buchholz relay used in conservator-type units.

Selection Criteria for Buyers

When specifying an oil immersed transformer for a distribution or industrial project, buyers should evaluate:

• Environmental exposure: humid, coastal, or dusty sites favor the hermetically sealed design because it isolates the oil from ambient moisture and contamination.
• Maintenance access: remote or low-maintenance-capacity sites benefit from the simpler hermetic structure that removes the breather and Buchholz relay servicing burden.
• Power rating range: confirm the required kVA rating fits within the manufacturer’s hermetically sealed product range, noting that this construction is commonly offered up to 2500 kVA.
• Substation type: for concrete kiosk substations, verify compatibility with ratings up to 1600 kVA, a common configuration in this segment.
• Cooling class: ONAN is standard; specify ONAF with fans if the load profile or ambient temperature demands additional forced-air cooling capacity.
• Protection scheme: confirm whether an electronic protection relay is included, particularly for units above 400 kVA.
• Winding material: copper or aluminum, selected based on load profile, cost target, and thermal performance requirements.
• Tap changer type: off-load or on-load tap changer, depending on whether voltage regulation must occur under energized conditions.

Efficiency and Service Life

Hermetically sealed oil-type transformers are widely regarded as the most popular choice for outdoor and industrial applications due to their robustness, high efficiency of approximately 99%, and long operational life. This efficiency level, combined with the reduced moisture and oxidation risk of the sealed design, gives utilities and industrial operators a dependable long-term asset with a lower total cost of ownership compared to open-oil designs requiring ongoing breather and relay maintenance.

Standards and Compliance

In Turkey, hermetically sealed transformers must comply with the TEDAŞ MLZ/99.032E technical specification. Buyers sourcing for export markets should confirm the applicable national or utility-specific technical specification for their project country, alongside the manufacturer’s own quality and testing documentation, since local grid codes and substation standards vary across the Middle East, Gulf, North Africa, CIS, and Sub-Saharan African markets.

Related guides

See also our guides on gas-insulated switchgear, ring main units and metal-clad switchgear.

What is a Transformer? Electrical Transformer Working Principle and Types

A transformer, also known as an electrical transformer or power transformer, is a static electrical device that transfers electrical energy from one voltage level to another. Transformers play a critical role in the generation, transmission, and distribution of electrical power.

An electrical transformer, commonly known as a power transformer or simply a transformer, is a static electrical device that transfers electrical energy from one voltage level to another through electromagnetic induction. Transformers are one of the most critical components in modern power systems because they enable efficient generation, long-distance transmission, and safe distribution of electrical energy.

Electricity is generated at power plants at relatively low voltages. To transmit this energy over long distances with minimal losses, the voltage must be significantly increased. At the consumption end, this high voltage needs to be reduced to safe and usable levels for homes, factories, and commercial buildings. This voltage conversion process is made possible by transformers.

What is a transformer and why is it so important? Without transformers, it would be impossible to transmit large amounts of electrical power economically over hundreds of kilometers or to safely deliver electricity to end users at appropriate voltage levels.

Working Principle of Transformer

The working principle of transformer is based on Faraday’s Law of Electromagnetic Induction. A transformer consists of two main windings — the primary winding (input side) and the secondary winding (output side) — wound around a common magnetic core made of laminated silicon steel sheets.

When an alternating voltage is applied to the primary winding, it creates a changing magnetic field in the core. This magnetic flux links with the secondary winding and induces an electromotive force (EMF). The voltage induced in the secondary winding depends on the turns ratio between the primary and secondary windings.

• If the secondary winding has more turns than the primary, the output voltage increases → step up transformer
• If the secondary winding has fewer turns than the primary, the output voltage decreases → step down transformer

Power remains approximately constant (P = V × I), while voltage and current change inversely. This fundamental relationship allows transformers to efficiently manage voltage levels throughout the power grid while minimizing energy losses.

Advantages of Transformers

Transformers offer several important advantages compared to other electrical machines:

• No moving parts, resulting in very high reliability and low maintenance
• Extremely high efficiency, often reaching 99% or higher
• Excellent voltage regulation
• Ability to handle very high power ratings
• Long operational life with proper maintenance

Because transformers have no rotating components, they experience significantly less mechanical wear and can operate continuously for decades with minimal intervention.

Types of Transformer

Transformers are classified in several ways depending on their construction, cooling method, and application.

According to Number of Phases:

• Single-phase transformers are commonly used in residential applications, small loads, and electronic circuits.
• Three-phase transformers are the standard in power generation, transmission, and industrial distribution systems due to their higher efficiency and power handling capability.

According to Cooling and Insulation Method:

• Dry type transformers use air or solid insulation. They are preferred in indoor locations such as hospitals, schools, shopping centers, and commercial buildings because they pose no fire or explosion risk.
• Oil type transformers use mineral oil or ester-based fluids for both insulation and cooling. They are widely used in outdoor substations, industrial plants, and high-power applications due to their superior cooling performance and higher power ratings.

According to Application:

• Power transformers are used in generation and transmission networks. They typically have ratings from 5 MVA up to several hundred MVA and operate at high voltage levels.
• Distribution transformers step down medium voltage to low voltage (usually 400V) for end consumers. They generally have ratings up to 5 MVA.
• Step up and step down transformers are used to increase or decrease voltage levels as required by the power system.
• Instrument transformers (current transformers and voltage transformers) are used for measurement and protection purposes.
• Special purpose transformers include isolation transformers, rectifier transformers, furnace transformers, and autotransformers.

Transformer Efficiency and Winding Materials

Modern electrical transformers achieve very high efficiency levels. The choice of winding material plays an important role in both cost and performance. While copper was traditionally preferred for its excellent conductivity, aluminum windings have become increasingly popular due to lower material cost and improved manufacturing technology. Today’s high-quality aluminum wound transformers can achieve performance levels very close to copper wound units.

Even small improvements in transformer efficiency have a major impact on overall system losses because thousands of transformers operate continuously across the grid.

Role of Transformers in Power Systems

Transformers are essential at every stage of the electrical power chain:

• At power plants, step up transformers increase generator voltage for efficient long-distance transmission.
• In transmission networks, power transformers maintain optimal voltage levels.
• In distribution networks, step down transformers reduce voltage to safe levels for consumers.
• In industrial facilities, specialized transformers supply the exact voltage and power quality required by sensitive equipment.

The electrical transformer remains one of the most important inventions in electrical engineering. Its simple yet highly effective working principle, combined with high efficiency and reliability, makes it indispensable in modern power systems. Whether it is a large power transformer in a transmission substation or a dry type transformer inside a commercial building, transformers enable the safe, efficient, and economical delivery of electrical energy from generation to consumption.

Understanding the working principle of transformer, different types of transformer, and their applications helps engineers and project owners make the right selection for their specific needs.

Related MV engineering guides

• Dry-Type vs Oil-Immersed Transformer Selection
• Transformer Sizing (kVA Selection) Calculation

Looking for Electrical Transformer for your project?

Looking for high-quality electrical transformer, power transformer, step down transformer, or complete solutions for types of transformer? We provide expert technical support for all your power distribution and industrial transformer needs.

For quotations and requests: info@electricistanbul.com

Whatsapp: +90 501 076 69 91